CIS (contact image sensor) has been intensively used in fax machines for scanning document image and converting it into electronic signals. FIG. 1 shows a CIS according to the prior art, which comprises a housing 1, a LED array 2, a scanning face glass 3, a lens 4, and a circuit board 5. The light source of LED array 2 emits light through the glass 3 onto document. Document reflects light onto the lens 4 permitting it to be further transmitted to the circuit board 5 causing it to produce a series of electronic signals for output. However, it is not satisfactory in function to use a LED array as the light source of a CIS. The drawbacks of using a LED array as the light source of a CIS are outlined hereinafter.
1) Because a LED array is complicated to manufacture and needs much installation space, causing the manufacturing cost of the CIS relatively increased.
2) Because various individual light emitting diodes are used to set up a LED array, any quality difference among the individual light emitting diodes will affect the uniformity of the intensity of light and, the damage of either light emitting diode will cause the scanning quality to be unstable.
3) The intensity of light is indirectly proportional to square distance. However, because a LED array occupies much installation space, the distance between the LED array of a CIS and document to be scanned cannot be greatly reduced. Therefore, the LED array of a CIS must consume much power so that the intensity of the attenuated light which was reflected by document is sufficient for an accurate detection.